The present invention relates to a system for managing the heat from a heat source like an electronic component. More particularly, the present invention relates to a system effective for dissipating the heat generated by an electronic component. Additionally, the present invention relates to a thermal interface sheet article used in conjunction with the heat management system of an electronic component.
With the development of more sophisticated electronic components, including those capable of increasing processing speeds and higher frequencies, having smaller size and more complicated power requirements, and exhibiting other technological advances, such as microprocessors and integrated circuits in electronic and electrical components and systems as well as in other devices such as high power optical devices, relatively extreme temperatures can be generated. However, microprocessors, integrated circuits and other sophisticated electronic components typically operate efficiently only under a certain range of threshold temperatures. The excessive heat generated during operation of these components can not only harm their own performance, but can also degrade the performance and reliability of the overall system and can even cause system failure. The increasingly wide range of environmental conditions, including temperature extremes, in which electronic systems are expected to operate, exacerbates these negative effects.
With the increased need for heat dissipation from microelectronic devices caused by these conditions, thermal management becomes an increasingly important element of the design of electronic products. As noted, both performance reliability and life expectancy of electronic equipment are inversely related to the component temperature of the equipment.
For instance, a reduction in the operating temperature of a device such as a typical SD silicon semiconductor can correspond to an exponential increase in the reliability and life expectancy of the device. Therefore, to maximize the life-span and reliability of a component, controlling the device operating temperature within the limits set by the designers is of paramount importance. A thermal management system is designed to assist with this objective.
One element of a thermal management system is a thermal interface material. A typical use for a thermal interface material is to thermally connect a computer chip to a cooling module (i.e., a heat sink as discussed above) to overcome contact resistance and lack of surface conformity between the heat sink, or the cooling module and the chip or other heat source. Typically, thermal interfaces consist of thermal greases, phase change materials and tapes. Flexible graphite is readily applicable to such applications because of its low thermal resistance and its ability to conform to the surfaces to be interfaced, especially when either or both surfaces are not completely flat. Such characteristics are important in a thermal management system because reducing the thermal resistance as much as possible is of paramount importance.
Although discussed in greater detail below, the process for manufacturing the flexible graphite used in the thermal interface is well-known and the typical practice is described in U.S. Pat. No. 3,404,061 to Shane et al., the disclosure of which is incorporated herein by reference. In general, flakes of natural graphite are intercalated in an acid solution. After the flakes are intercalated they are washed and dried and then exfoliated by exposure to a high temperature for a short period of time. This causes the flakes to expand or exfoliate in a direction perpendicular to the crystalline planes of the graphite. The exfoliated graphite flakes are vermiform in appearance and are therefore commonly referred to as worms. The worms may be compressed into sheets or foils with a density approaching theoretical density although a density of about 1.1 g/cc is considered typical for most applications. The sheets of flexible graphite can be cut into any desired configuration to suit a particular application.
Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as graphene layers or basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size: the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites possess anisotropic structures and thus exhibit or possess many properties that are highly directional such as thermal and electrical conductivity.
Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the xe2x80x9ccxe2x80x9d axis or direction and the xe2x80x9caxe2x80x9d axes or directions. For simplicity, the xe2x80x9ccxe2x80x9d axis or direction may be considered as the direction perpendicular to the carbon layers. The xe2x80x9caxe2x80x9d axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the xe2x80x9ccxe2x80x9d direction. The graphites suitable for manufacturing flexible graphite sheet, typically natural graphite flake, possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the xe2x80x9ccxe2x80x9d direction, and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
Graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or xe2x80x9ccxe2x80x9d direction dimension which is as much as about 80 or more times the original xe2x80x9ccxe2x80x9d direction dimension can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, foils, mats or the like (typically referred to as xe2x80x9cflexible graphitexe2x80x9d). The formation of graphite particles which have been expanded to have a final thickness or xe2x80x9ccxe2x80x9d dimension which is as much as about 80 times or more the original xe2x80x9ccxe2x80x9d direction dimension into integrated flexible sheets by compression, without the use of any binding material, is believed to be possible due to the mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal and electrical conductivity due to orientation of the expanded graphite particles and graphite layers substantially parallel to the opposed faces of the sheet resulting from very high compression, e.g. roll pressing. Sheet material thus produced has excellent flexibility, good strength and a very high degree of orientation.
Briefly, the process of producing flexible, binderless anisotropic graphite sheet material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a xe2x80x9ccxe2x80x9d direction dimension which is as much as about 80 or more times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles that generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material can be within the range of from about 0.04 g/cc to about 2.0 g/cc. The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon roll pressing of the sheet material to increase orientation. In roll pressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the xe2x80x9ccxe2x80x9d direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the xe2x80x9caxe2x80x9d directions and the thermal and electrical properties of the sheet are very different, by orders of magnitude, for the xe2x80x9ccxe2x80x9d and xe2x80x9caxe2x80x9d directions.
U.S. Pat. No. 6,245,400 to Tzeng et al. discloses a release lined pressure sensitive adhesive flexible graphite sheet article that has low thermal resistance properties. The flexible graphite sheet comprises a flexible graphite substrate, and adhesive primer coating thereon, a pressure sensitive adhesive coating on the adhesive primer coating and a release liner applied to the pressure sensitive adhesive coating.
An object of the present invention is to improve the conduction of heat between a heat source and a cooling module.
Another object of the present invention is to provide a thermal interface that has reduced contact resistance and increased thermal conductivity with respect to the heat dissipating component and the heat source.
More specifically, an embodiment of the present invention is a thermal interface material that comprises a flexible graphite sheet article, the sheet article containing oil.
Another embodiment of the present invention is a thermal management system that comprises a heat source having an external surface; a thermal interface which comprises a flexible graphite sheet article that contains oil, the thermal interface being in operative communication with the external surface of the heat source; and a heat dissipating component having a heat collection surface and a heat dissipation surface, the heat collection surface being in operative communication with the thermal interface. In this embodiment, arranging the heat collection surface in operative connection with the thermal interface causes dissipation of heat from the heat source.
Another embodiment of the present invention is a thermal management system that comprises a heat source that comprises an electronic component, a thermal interface, and a heat sink. In this embodiment, the heat source has an external surface; the thermal interface comprises a flexible graphite sheet article having two parallel planar surfaces extending in a direction parallel to the planar direction of the crystal structure of the graphite in the sheet, a first of the planar surfaces of the thermal interface being in operative contact with the external surface of the heat source. Furthermore, the graphite sheet preferably contains at least 2% by weight of oil. In this embodiment, the planar area of the first planar surface of the thermal interface is greater than the area of the external surface of the heat source. Finally, the heat sink has a heat collection surface and at least one heat dissipation surface. The heat collection surface of the heat sink is in operative contact with the second of the planar surfaces of the thermal interface.
Another embodiment of the present invention is a process for preparing a thermal interface material. The process comprises providing a flexible graphite sheet material that has two parallel planar surfaces extending in a direction parallel to the planar direction of the crystal structure of the graphite in the sheet, and providing oil; and contacting said oil with the graphite sheet until from about 2% to about 75% by weight of the oil is absorbed into the thermal interface material.
Finally, another embodiment of the present invention is a method of dissipating heat from a heat source comprising using the thermal interface materials and the thermal management systems of the present invention.
Preferably in the above embodiments, the thermal interface comprises an anisotropic flexible graphite sheet having a planar area greater than the area of the external surface of the heat source. Furthermore, preferably in the above embodiments, the oil is mineral oil.